This invention relates to protective coatings for metal alloy components exposed to high temperature gas environments and severe operating conditions, such as the working components of gas turbine engines used in electrical power generation. More particularly, the invention relates to a new type of thermal barrier coating (xe2x80x9cTBCxe2x80x9d) for use in gas turbine engines and a method for applying the new TBC coatings to metal substrates. The coating consists of a thermal insulating ceramic layer whose composition and deposition significantly enhance the erosion resistance of the turbine components while maintaining a spallation resistance equivalent to or better than conventional coatings. The preferred coating composition is applied using a dense vertically cracked vapor deposition process.
The operating conditions to which gas turbine hardware components are exposed can be thermally and chemically severe. Thus, by necessity the surfaces of the metal substrates used to form turbine, combustor and augmentor components must exhibit greater than average mechanical strength, durability and erosion resistance in a very hostile, high temperature gas environment. As used herein, the term xe2x80x9cerosionxe2x80x9d refers to the process whereby a surface, particularly metal, is bombarded by contaminant particles of sufficiently high energy that cause other particles to be ejected (eroded) from the surface, resulting in degradation and cracking of the substrate material.
In recent years, significant advantages have been achieved by using high temperature alloys in gas turbine systems by incorporating iron, nickel and cobalt-based superalloys in coatings applied to the substrate of key turbine components. The purpose of an effective surface coating is two-fold. First, the coating must form a protective and adherent layer that guards the underlying base material against oxidation, corrosion, and degradation. Second, the coating should have low thermal conductivity relative to the substrate. As superalloy compositions have become more complex, it has been increasingly difficult to obtain both the higher strength levels that are required (particularly at increased gas turbine operating temperatures) and a satisfactory level of corrosion and oxidation resistance. The trend towards higher gas turbine firing temperatures has made the oxidation, corrosion and degradation problems even more difficult. Thus, despite recent improvements in thermal barrier coatings, a significant need still exists for more effective, less degradable high temperature coatings since most alloy components cannot withstand the long service exposures and repetitive cycles encountered in a typical gas turbine environment.
Many of the known prior art coatings used for gas turbine components include aluminide, MCrAlY and ceramic components. Typically, ceramic coatings have been used in conjunction with a bond coating formed from an oxidation-resistant alloy such as MCrAlY, where M is iron, cobalt, and/or nickel, or from a diffusion aluminide or platinum aluminide that forms an oxidation-resistant intermetallic. In higher temperature applications, these prior art bond coatings form an oxide layer or xe2x80x9cscalexe2x80x9d that chemically bonds to the ceramic layer to form the final bond coating.
In the past, it has also been known to use zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides as the primary constituent of the ceramic layer. Yttria-stabilized zirconia (hereafter xe2x80x9cYSZxe2x80x9d) is often used as the ceramic layer for thermal bond coatings because it exhibits favorable thermal cycle fatigue properties. That is, as the temperature increases or decreases during gas turbine start up and shut down, the YSZ is capable of resisting stresses and fatigue much better than other known coatings. Typically, the YSZ is deposited on the metal substrate using known methods, such as air plasma spraying (xe2x80x9cAPSxe2x80x9d), low pressure plasma spraying (xe2x80x9cLPPSxe2x80x9d), as well as by physical vapor deposition (xe2x80x9cPVDxe2x80x9d) techniques such as electron beam physical vapor deposition (xe2x80x9cEBPVDxe2x80x9d). Notably, YSZ deposited by EBPVD is characterized by a strain-tolerant columnar grain structure that enables the substrate to expand and contract without causing damaging stresses that lead to spallation. The strain-tolerant nature of such systems is now documented in the literature.
Stabilization with yttria serves to prevent zirconia from undergoing a tetragonal to monoclinic phase transformation at about 1000xc2x0 C. that would otherwise result in detrimental volume expansion and eventual coating failure. In the mid-1980s, Stephan Stecura at NASA determined that zirconia stabilized with 7 weight % yttria (xe2x80x9c7 YSZxe2x80x9d) was the best composition for spallation resistance on a superalloy substrate. (See U.S. Pat. No. 4,485,151). Stecura concluded that 6-8 weight % yttria stabilized zirconia (xe2x80x9c6-8 YSZxe2x80x9d) was optimal when the coating was applied using air plasma spraying.
Thus, since the mid-to-late 1980s, conventional practice in the art has been to xe2x80x9cpartially stabilizexe2x80x9d zirconia with at least 6-8 weight % yttria. The Stecura ""151 patent teaches against using lesser amounts of yttria since the zirconia is described as being only xe2x80x9cpartiallyxe2x80x9d stabilized to provide an optimum mixture of cubic, tetragonal and monoclinic phases of coating material. Thus, historically those skilled in the art have considered the 6-8 percent level of yttria recommended by Stecura as the lowest effective amount that would produce an acceptable coating capable of demonstrating sufficient spallation resistance under the extreme operating conditions of gas turbine engines.
More recently, an improved thermal ceramic layer for use in hostile thermal environments has been developed by The General Electric Company formed from zirconia stabilized by yttria. The ceramic is characterized by a columnar grain structure in which a monoclinic phase is present. Commonly assigned U.S. Pat. No. 5,981,088 discloses using about 2 to 5% by weight yttria to stabilize the zirconia, with the coating being deposited on the substrate using electron beam physical vapor deposition (xe2x80x9cEBPVPxe2x80x9d). The ceramic coatings described in the ""088 patent have been found particularly beneficial for use on aircraft engine components that must withstand a high number of thermal cycles.
Despite the recent developments in coatings summarized above, there remains a need in the art for an improved zirconia-based coating that is optimal for use in forming protective coatings on metal alloy components exposed to high temperature environments in gas turbine engine components used for electrical power generation. The need also exists for improved methods of applying such coatings to key turbine components exposed to hostile chemical conditions at high temperatures. That is, a need still exists for improved yttria-stabilized zirconia coatings that have strong chemical and erosion resistance when exposed to very hot exhaust gases, while maintaining a spallation resistance comparable to conventional systems such as those taught by Stecura.
The present invention meets the above needs by providing a new thermal insulating ceramic layer for use in a thermal barrier coating system on metal alloy components designed for use in a hostile thermal environment. Components that are well-suited for coating are nozzles, buckets, shrouds, airfoils, and other combustion hardware found in the hot gas paths of gas turbine engines. The coatings of the present invention tend to reduce the temperature at the surface of the metal alloy because the thermal conductivity of the coating is an order of magnitude lower than that of the metal substrate. Only a thin layer of ceramic is required to reduce the heat flux to a metal when a thermal gradient exists (5-50 mils). The temperature at the surface of the metal can be up to 400xc2x0 F. lower than the temperature at the surface of the ceramic coating. The ceramic layers are particularly suited to applications where the gas temperature is in excess of 1000xc2x0 C. and/or where severe thermal cycle fatigue stresses exist due to repeated start ups and shut downs of the gas turbine.
Surprisingly, it has now been found that an improved thermal insulating ceramic layer in accordance with the invention can be formed of zirconia that has been partially stabilized by yttria in the amount substantially lower than predicted by Stecura, namely about 4 weight % (referred to herein as xe2x80x9c4 YSZxe2x80x9d). For the first time, it has also been found that the yttria-stabilized zirconia can be applied using a dense vertically cracked (xe2x80x9cDVCxe2x80x9d) vapor deposition process. Using the DVC process, the yttria transforms the ceramic into a tetragonal crystal structure that resists volume changes during repeated thermal cycling. The stabilized transformation also toughens the zirconia when mechanical stresses are applied. Preferably, the zirconia formed using the DVC process includes 0-1% by weight Hafnia (HfO2) in solid solution.
Thermal barrier coating systems that incorporate a ceramic layer in accordance with the invention have been found to exhibit increased erosion resistance, chipping resistance, and prolonged cycle life. The ceramic layer is also significantly less expensive because of the reduced amount of yttria used to stabilize the zirconia. Other objects and advantages of this invention will be better appreciated from the detailed description that follows.